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Abstract

The use of non-indigenous commercial arbuscular mycorrhizal fungi as bio-fertilizers is increasing worldwide without a clear understanding of the persistence and consequences on the indigenous AMF communities and crop productivity. To address this research gap, a three-year field incubation study using open-ended soil cores transplanted to four sites in Saskatchewan was initiated in 2011. A growth chamber study was also carried out in 2014 to examine the impact of AMF inoculants of different origins on the alteration of indigenous AMF communities and subsequent crop growth performance of lentil (Lens culinaris L.), chickpea (Cicer arietinum L.), and field pea (Pisum sativum L.).
Non-indigenous Rhizophagus irregularis inoculant was applied into soil cores in which field pea-wheat-field pea were subsequently grown in three consecutive cropping seasons (2011 to 2013). The 18S rRNA gene pyrosequencing data from trap roots of field pea revealed that a single application of the commercial inoculant persisted in roots competing with indigenous AMF over three crop seasons in two of the four sites and declined in the remaining two sites and was undetectable by the third cropping season. Inoculation resulted in a significant alteration of the resident AMF communities and suppression of some indigenous AMF taxa that were low in abundance (Septoglomus, Archaeospora, Diversispora and Entrophospora). Inoculation was one of the significant driving factors regulating the composition and diversity of indigenous AMF communities.
Phylogenetic analysis using pyrosequencing was efficient in detecting and quantifying the relative abundance of AMF and discriminated between introduced and indigenous AMF taxa in roots. Locally isolated Semiarid Prairie Agricultural Research Centre (SPARC) AMF inoculant strain, F. mosseae B04 significantly enhanced shoot N and P uptake and biomass in pulse crops with minimum disturbance to resident AMF communities in roots compared to commercial inoculant strain, R. irregularis 4514535. Inoculation with Glomeromycota In-vitro Collection (GINCO) inoculant strain, F. mosseae DAOM 221475 also enhanced N uptake in chickpea; however, uptake of P and biomass response were variable between crops. Strong positive correlations existed between the relative abundance of major indigenous AMF taxa (Rhizophagus and Funneliformis) and shoot N, P uptake and biomass production of lentil, chickpea and pea. Growth performances were mediated by the influence of indigenous AMF taxa as a consequence of inoculation by inoculant that was locally isolated. Assessment of pyrosequencing data with pooled versus non-pooled replicated trap root samples (2011 and 2013 crop seasons) prior to DNA extraction showed that the relative abundance of major (highly abundant) indigenous AMF genera was similar in both sampling strategies. Abundance of minor (low abundant) AMF genera was significantly reduced and was undetectable in some root samples as a consequence of pooling replicates. Pooling replicates reduced the cost of analyses and reduced efforts significantly but it compromised estimates of AMF community composition and diversity.
These results raised several questions such as 1) does inoculant anastomose genetically with different individual strains, 2) how does genetic manipulation impact rhizosphere microbial communities and subsequent plant growth and productivity, 3) what are the important determinants for the survival of introduced inoculants, 4) does inoculation have direct or indirect impact on growth performance, etc. All these relevant questions regarding the mechanism and nature of competition between indigenous and non-indigenous AMF taxa in different crops, soils, climates and subsequent crop productivity over long-term warrant further investigation.